Full Scale Reynolds Number Simulation

Introduction

Reynolds number along with Mach number is one of the most important parameters used to measure the airflow around an aircraft model. It is closely related to the scale of the model being tested and can have a significant effect on the structure of the airflow around the model which in turn can affect all aspects of the measured aircraft performance.

Aircraft models are small scale versions of the real aircraft and as a result, the Reynolds number of the airflow around the model is less than what the real aircraft experiences in flight. To ensure that the wind tunnel test data is as accurate as possible, the airflow over the model surface can be artificially altered to simulate a higher Reynolds number.  This is done by the appropriate positioning and sizing of 'roughness bands' to transition the boundary layer flow from laminar to turbulent; see below for more information. 

The science of artificially simulating higher Reynolds numbers is referred to as Scale Effects.  ARA's first ever Chief Aerodynamicist was Barry Haines, a World renowned specialist in Scale effects who determined the practices for fixing roughness bands that are still carefully adhered to at ARA today.  Barry was awarded an O.B.E by Queeen Elizabeth II and the Gold Medal of the Royal Aerospace Society for his distinguished work in this field.

Boundary Layer

The boundary layer is a very thin sheet of slow moving air right next to the model surface; the behaviour of this layer can affect all aspects of aircraft performance such as fuel economy or stability and control.

The nature of the boundary layer tends to be either laminar (like smooth sheets sliding over each other) or turbulent (with tiny vortices mixing the fast moving air at the edge of the boundary layer with slow moving air at the model surface).

Where the boundary layer changes from laminar to turbulent is strongly influenced by the Reynolds number which in turn is governed by the model scale.

Roughness Bands

Roughness bands are used to change the nature of the boundary layer from laminar to turbulent at particular locations on the model surface. The bands are made from tiny glass beads embedded in a thin strip of glue. The beads introduce disturbances into the boundary layer which “trip” the flow changing it from laminar to turbulent thereby simulating a full-scale Reynolds number.

Sublimation Tests

In order to verify that the roughness band is working, a sublimate is sprayed onto the model which evaporates in regions of turbulent flow but remains on the model surface in areas of laminar flow.

The photograph on the right shows a close-up image of a nacelle with a roughness band (tinted blue) installed. In front of the band, the white powdery looking substance indicates the region of laminar flow. The absence of this sublimate behind the band shows that the roughness band has worked and is successfully tripping the boundary layer to turbulent flow.

Transition Sweeps

Different Reynolds numbers can be simulated by varying the position of the roughness bands – this technique is known as a “Transition Sweep”.